1. Field of the Invention
The present invention generally relates to a lighting device including a plurality of light-emitting diodes (hereafter abbreviated as xe2x80x9cLEDxe2x80x9d).
2. Related Background Art
As compared with conventional light sources such as a filament lamp and a halogen lamp, LEDs have the advantage of high reliability and long life. Further, since the luminous efficiency of LEDs has been improved recently, LEDs are expected to be used as alternatives for these conventional light sources.
LEDs emit light of only specific wavelengths because of their light-emitting principle. Therefore, in order to obtain white light using LEDs, an LED emitting blue light has to be combined with a phosphor which can be stimulated by the light emission of the LED to emit yellow-green light, as described in JP 2000-208815A. Alternatively, as described in JP11(1999)163412A, a plurality of LEDs that emit colored lights such as red, blue, and green lights have to be combined with one another.
However, the former method employing a phosphor has the following problems. That is, wavelength conversion necessarily degrades the luminous efficiency. Accordingly, this method is not preferable in terms of efficiency. In addition, since a color of emitted light (hereafter called xe2x80x9cluminescent colorxe2x80x9d) is uniquely determined depending on the combination of wavelengths of light emitted from the LED and the phosphor, it is impossible to control the color. Thus, if there is a change from the initial color tone due to degradation of the phosphor or the like, the color tone cannot be corrected. Further, since the film thickness of the phosphor, the output power and the wavelength of emitted light from the LED vary to some extent in the manufacturing process, it is difficult to make the luminescent color constant.
With the latter method using LEDs having a plurality of luminescent colors, higher luminous efficiency can be realized and the luminescent color can be controlled. However, since the LEDs for different colors are made of different compositions or different materials, the temperature dependence of their output powers and the deterioration rates are different from one another. As a result, the color tone would change depending on the operating conditions.
JP 10(1998)-49074A describes the method for coping with the problems as above, where LEDs are used as a light source for a backlight in a color display device. More specifically, an optical sensor is used for detecting the luminance level of each colored light emitted from the LEDs, so that each luminance level can be controlled in response to the value detected by the optical sensor, which allows a constant luminescent color to be produced.
However, this method is easily applicable to devices such as a color display device whose light quantity is relatively small, but cannot be applied simply to devices such as a lighting device to which the present invention relates. This is because lighting devices require more LEDs than color display devices and so require a structure capable of outputting a detection result incorporating light emitted from a number of LEDs so as to adequately drive the LEDs.
A higher degree of detection accuracy can be obtained by providing each LED with an optical sensor. But, such a construction is impractical, because it makes the device large and the cost expensive. To cope with this problem, the above-mentioned JP 10(1998)-49074A describes one optical sensor that is shared among different luminescent colors. However, this method does not provide a construction suitable for outputting a detection result incorporating all of the light emitted from a number of LEDs, in the case where the LEDs are arranged in a dispersed manner as in the light source for a lighting device.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a lighting device where a small number of photodetectors detect the light intensity incorporating light emitted from a plurality of LEDs and the driving of each LED is controlled based on the detected signals, so that even in the case where each LED has different light-emission characteristics, a predetermined light-emission state can be obtained.
The lighting device according to the present invention includes: a plurality of LEDs that are arranged in an at least two-dimensionally dispersed manner; a transparent resin layer that covers the plurality of LEDs in an integrated form; a photo-detecting unit using a photodetector that detects an intensity of light emitted from the plurality of LEDs, the photodetector being arranged inside, on a surface, or in the vicinity of the transparent resin layer; and a power supply circuit unit that controls the driving of the plurality of LEDs based on a detection output from the photo-detecting unit. Here, the number of the photodetectors is smaller than the number of the LEDs, and the photodetector detects an intensity of light emitted from the LEDs and propagated through the transparent resin layer.
With this construction, light emitted from the plurality of LEDs is propagated through the transparent resin layer and detected by the photodetector. Consequently, even with a number of LEDs, this enables the photodetector smaller in number to perform detection incorporating light emitted from the LEDs. Therefore, optical output from the device can be kept constant for a long time.
In the aforementioned construction, the plurality of LEDs may be mounted on a surface of a substrate by bare-chip mounting, and the plurality of LEDs and the substrate may be covered with the transparent resin layer.
In addition, in the above construction, preferably, when the top surface of the substrate and the surface of the transparent resin layer are approximately parallel with each other, a thickness h of the transparent resin layer satisfies:
h greater than d/(2xc3x97tan(arc sin(1/n)),
where d and n represent a maximum distance between two LEDs out of the plurality of LEDs and a refractive index of the transparent resin layer, respectively.
This relationship prevents the light emitted from one of the LEDs from being incident on another LED and absorbed therein. As a result, the efficiency of outputting the light from the device can be improved, while the amount of light incident upon the photodetector can be increased.
The lighting device further may include: a recess formed on the surface of the substrate; and a metal film applied over the surface of the substrate. Here, the walls of the recess may be inclined so as to form a reflector made of the metal film, the plurality of LEDs may be mounted on a bottom of the recess, and the substrate including the recess may be covered with the transparent resin layer.
In addition, in the aforementioned construction, the LEDs may be composed of a plurality of groups of LEDs, the groups emitting different colors of lights, the photo-detecting unit may detect an intensity of light emitted from the LEDs as to each color, and the control circuit may control the driving of the LEDs so that the LEDs have a predetermined balance of light intensities of the colors according to an output detected as to each color by the photo-detecting unit.
In the above construction, the photo-detecting unit may include a photo-detecting device for each color, a sensitivity to light of the photo-detecting device conforming with a peak wavelength of a corresponding color of light. With this construction, the light intensity of each group of LEDs (i.e., for each color) can be detected.
Further, in the above construction, the plurality of LEDs may be turned on sequentially by color, and the photo-detecting unit includes a photodetector smaller in number than the colors of lights, the photodetector detecting light in synchronization with the sequential lighting timings, so that the photodetector is shared in detecting the plurality of colors of lights. In this construction, by turning on the LEDs sequentially by the group (i.e., by color), for example, in the order of red, green, and blue, and monitoring the output voltage from the photo-detecting device with the same timing, the intensity ratio of one color to another color can be obtained. Then, the driving of the LEDs is controlled so as to keep the ratio at the predetermined value, whereby a desired color tone and a constant light intensity can be obtained.
In addition, in the above construction, a distance between two LEDs among those emitting the same color of light and concurrently turned on preferably is set longer than a distance between adjacent LEDs in an array of the plurality of LEDs. When emitting light, the LEDs also generate heat. Therefore, by making the distance between the LEDs concurrently turned on longer, the thermal influence of one LED upon another can be reduced, and so heat generated when mounting a plurality of LEDs as a package can be decreased.
In the aforementioned construction, an antireflective coating preferably is applied over the surface of the transparent resin layer. The antireflective coating made of MgF2 or the like allows the light propagated inside of the resin to be effectively taken to the outside.
Further, in the aforementioned construction, preferably, the photodetector and the plurality of LEDs that are incorporated integrally in the transparent resin layer and the power supply circuit are mounted on a same substrate. By mounting the light source unit and the circuit part that controls the light source unit on the same substrate, it is possible to provide the lighting device in an integrated, miniaturized, and thinner configuration.